One of the first choices you are faced with in Warcraft is what server to roll on. My first character, a warrior, was on a PvP server--mainly because other friends were playing there--then Coriel was made on a PvE server, and recently I've been levelling a warlock on the PvP server.
I have a kind of love-hate relationship with world PvP. Before I started playing WoW, I used to be a pretty hard-core PvP type of guy, even to the point of advocating perma-death. After all, if there are no consequences, what is the point of playing in a persistent world? Now, however, I've moved far away from that position, and probably into the "carebear" side of things.
In my view, a PvP server offers much higher highs, but also much lower lows.
My single favorite memory of WoW is a world PvP memory. My warrior and a priest friend were around level 30 and adventuring in Durnholde Keep in Hillsbrad at night. A level 30 rogue snuck up behind me, sapped me, and attacked the priest. She killed the priest, and I came out of the sap and charged her. She took off, leaping from roof to roof on the buildings in the Keep. We had a crazy, moonlit chase, jumping across rooftops, which ended when I mistimed a jump and fell off a roof, allowing her to make her escape.
But I remember thinking to myself, as I was leaping, that this was awesome. It was a good fight, against a skilled opponent of the same level. The chase was like something from a movie. This was an encounter that simply could not happen on a PvE server.
But then you have to balance that experience against all the times a 60 rogue has decloaked inside a town and slaughtered me. And then proceeded to corpse camp my body. Higher highs, but lower lows.
A PvE server on the other hand, is much more restful. On a PvP server, you always have to have your guard up, watching for the enemy. It's a very tiring playstyle. Sometimes it is nice to be able to quest in peace. As well, I like concentrating on one task at a time, and PvP interrupts that, making it annoying. Finally, a fair fight on a PvP server is rare, and something to be treasured. Realistically, the vast majority of world PvP consists of a high level character ganking a low level character.
So if I had to chose between a PvE or PvP game, I'd probably choose to play the PvE game. But I'm glad that WoW offers the choice.